Download Clusters

Stellar Clusters
• Homework Problems Chapter 13
– Review Questions: 1-3, 9-11
– Review Problems: 1, 2, 7
– Web Inquiries: 1, 4
H-R Diagram
An H-R diagram reveals three
significant features.
• The Main Sequence
– core hydrogen fusion
L/Lsun
• Red Giants
– hydrogen shell source
– (core helium fusion)
• White Dwarfs
– carbon-oxygen core leftovers
This is a snapshot in the life of many,
unrelated stars.
O
B
A
F
G
K
M
H-R Diagram
Other areas of the H-R diagram are
sparsely populated either because
stars just don’t exist with those
specific combinations of
L/Lsun
luminosity and
temperature, or because the
stars evolve so quickly and
spend so little time in that area
that the probability of seeing
stars located there is very low.
(Example: “Hertzsprung Gap”)
O
B
A
F
G
K
M
Clusters Formation
•
•
•
•
Stars from the same formation epoch (same “birthday”)
Same Age
Similar Chemical Composition
Same Distance from the Earth
Stellar Clusters
• OPEN Cluster
–
–
–
–
Bright
Hot, Young stars
Thousands of stars
Loose Associations
• GLOBULAR Cluster
–
–
–
–
Fainter
Cooler, Older stars
Hundreds of Thousands of stars
Tightly Bound Systems
Double Cluster
• OPEN Cluster
Very few evolved
stars.
Globular Cluster
Many evolved stars.
Cluster HR Diagrams
H-R diagrams
revealing
a variety of
ages among
clusters.
Who is the youngest?
Who is the oldest?
Young Cluster
Young Cluster HR Diagram
Cluster Evolution
Combined Cluster H-R diagrams
Hyades
M44 Cluster
M41 Cluster
M2001 Cluster
Globular Cluster M5
Main Sequence Turnoff
Since stellar evolution
is highly dependent
upon the mass of a star,
the turn-off point
of a clusters
main-sequence
is a measure of the
clusters age.
Horizontal Giant Branch
Pulsating Variable Stars
Instability Strip
The Pulsating Stars
Cepheid Pulsation Sequence
Pulsating Variables
Light curves for two
different Cepheid
variables showing
the periodic changes
in brightness.
Cepheids
Cepheid Pulsation
The interplay
of Radius
and Temperature.
L = 4pR2sT4
Period - Luminosity Relation
Cepheid Period - Luminosity
Observe apparent magnitude (m)
Observe Period of pulsation (P)
Utilize P-L relation to infer absolute magnitude (M)
Use distance modulus to
obtain distance:
m-M = 5 log D - 5
SMC and LMC
The Milky Ways two irregular satellite galaxies.
Distance Determinations
Distance Indicators